Optical disk recording and archiving schemes are becoming more common, and find use when large amounts of information are to be stored and retrieved. The amount of data which can be stored, and the data transfer rates, are great enough so that video can in some cases be recorded and played back in real time. In other contexts, multichannel data recorders may be used for recording large amounts of information in a short period of time, as for example during planetary flybys by exploratory spacecraft or during airborne or satellite military reconnaissance missions.
During optical disc recording, the disk, which may be removable, is supported by a turntable, which is rotated relative to an optical head. FIG. 1 is a simplified representation of an optical disk apparatus 10 useful for recording information and playing it back. Such arrangements are often called simply "recorders" or "players" even though their principal use may be playback or recording, respectively. In FIG. 1, an optical disk 12, which may be a magnetooptical disk, is supported for rotation about a shaft 16 by a turntable 14, and the turntable, and therefore disk 12 lying thereon, is rotated by a motor 18. A tracking head, designated generally as 20, includes a light source, such as a laser diode 22, which generates a light beam 24. Light beam 24 enters a light modulator illustrated as a block 26. Modulator block 26, when recording is to be accomplished, modulates the light beam coupled thereto by laser diode 22. The modulation information is coupled to light modulator 26 from a source (not illustrated) by way of a data path 25. As an alternative to the combination of laser diode 22 and data modulator 26, the laser disk may itself be modulated by the data, in known fashion.
Light modulator 26 of FIG. 1 passes information-modulated light beam 48b, to a controllable mirror 30, which reflects the light beam to form reflected light beams 48b, directed toward the sensitive surface of optical disk 12. The light spot on the disk which results from beam 48b is represented by dot 50. Rotation of turntable 14 and disk 12 in the direction indicated by arrow 9 results in the tracking of spot 50 along a continuous spiral path on the disk, some of the turns of which spiral are illustrated, in part, by dashed paths 52a, 52b, and 52c.
Mirror 30 of FIG. 1 is hinged about a shaft 32, so that the mirror may be moved about the shaft by a motor or actuator 34, controlled by a tracking servosystem including a reflected light detector 56 and a servo control block 58. In tracking operation during either recording or playback, light reflected by the surface of disk 12 in response to light spot 50 is coupled by means (not illustrated) to light detector 56 as light beam 54. Detector 56 produces, on signal path 57, signals representative of the amplitude of the reflected beam, which in the case of a disk prerecorded with tracking pads as described below in conjunction with FIG. 2b, will include signals which represent the amplitude of beams 48 reflected from tracking pads 214 and 212, respectively. The detector output signal on signal path 57 may also include sensed data, which is made available to an output port (not illustrated). Servo block 58 of FIG. 1 processes the detected signals from signal path 57, by processing which may include synchronous gating, and may also include integration or averaging, and applies the resulting signals to a subtracting circuit, such as circuit 320 described in conjunction with FIGS. 3b and 3d. The subtracted signal may be further processed in servo block 58 of FIG. 1, as by amplification. The resulting control signal is coupled by a path 59 from servo block 58 to mirror actuator 34, which rotates the mirror in response to the control signal to direct the reflected beams 48 radially inward or outward, corresponding to "left" or "right" tracking, respectively. In this manner, a degenerative feedback loop is closed, by which the beam tends to follow the sensed track path (track).
FIG. 2a illustrates portions of three adjacent unrecorded tracks 210a, 210b, and 210c, termed track paths herein, from among a large number of concentric turns of a single elongated optical track path on an optical disk. A track path is, therefore, the path which the recording light beam spot should follow as it traverses the disk. If the optical disk on which the unrecorded track paths of FIG. 2a are to be recorded has no actual tracking information, there may be no optical difference between the desired track path 210 and the nontrack spaces 208 between the track paths. In the absence of a predefined path on the disk, therefore, the recording path must be established by the recorder itself. To avoid this complexity, the unrecorded disk is often preformatted with a discernible track path.
As illustrated in FIG. 2b, a common form of a discontinuous track path preformatting consists of tracking pads 212 and 214 adjacent to, and spaced along the track path 210, and on either side thereof. More particularly, in FIG. 2b the outermost turn 210a of the illustrated track path turns has spaced-apart tracking pads 212a, 212b, and 212c adjacent to the track path, and somewhat overlapping onto the outermost edge of the track. The next turn closer to the center of the disk, namely turn 210b, has spaced-apart tracking pads 212m, 212n, and 212o adjacent to, and somewhat overlapping onto the outer edge of the turn. The innermost illustrated turn 210c of the track path has spaced-apart tracking pads 212r, 212s, and 212t adjacent to, and overlapping onto the outermost edge. Similarly, the illustrated turns 210a, 210b, and 210c have spaced-apart tracking pads 214a, 214b, 214c, 214m, 214n, 214r, and 214s adjacent to, and overlapping onto the interior edges of the track path.
When recording is to be accomplished on an optical disk preformatted as described in conjunction with FIG. 2b, the optical head of the arrangement of FIG. 1 produces a recording beam spot (spot) 310, as illustrated in FIG. 3a, impressed with the information or data to be recorded. Beam spot 310 periodically falls onto the exterior and interior tracking pads 212 and 214, respectively, during tracking. Spot 310 moves to the right in FIG. 3a, in the direction of arrow 316, relative to the disk and the track paths thereon. When tracking is correct, with recording spot 310 centered on track path 210b, some of the light of spot 310 periodically overlaps the innermost edges of tracking pads 212, but does not fully cover tracking pads 212 during its transit or traversal. Similarly, during correct tracking, some of the light of spot 310 overlaps the outermost edge of tracking pads 214 during its transit, but does not fully cover tracking pads 214. As a result, the light reflected from each of the tracking pads is not at a maximum during correct tracking, because light spot 310 does not fall completely onto either tracking pad. The light reflected by the two tracking pads 212, 214 is individually synchronously detected by the servo system of the arrangement of FIG. 1 by use of a clock signal derived from the tracking pads to form two tracking signals, and the two tracking signals are applied to a subtracting circuit, which is part of the tracking servo system by which the optical tracking head is controlled to keep the light beams on the desired tracks. When tracking is correct, as illustrated in FIG. 3a, the light reflected by outermost tracking pads 212 and that reflected by innermost tracking pads 214 will be approximately equal.
FIG. 3b illustrates a subtracting circuit 320, which may be part of a servo system by which tracking is accomplished. Subtracting circuit 320 subtracts signals of equal magnitude, representing the equality of the two reflected beams during correct tracking, as illustrated in conjunction with FIG. 3a. The magnitude of each signal input is indicated as having the representative value of 1/2 or 0.5. The output signal of subtracting circuit 320 is therefore 1/2-1/2, which is zero. This output signal drives the tracking servosystem by which the light beam is deflected. With correct tracking, the drive signal at the output of subtracting circuit 320 is zero, indicating that no tracking correction is required.
In FIG. 3c, light spot 310 falling onto track path 210b is illustrated as being offset toward the center of the disk, or to the "right" for the indicated direction 316 of motion of the beam set relative to the disk. This offset constitutes a mistracking, in that the recording (or playback) beam is offset away from the center of the track path 210b. As a result of this mistracking, light spot 310, when it transits tracking pads 212, overlaps less than when tracking is correct. Consequently, the amount of light reflected by the tracking pads 212 as a result of transversal by light spot 310 is reduced. Similarly, light spot 310 overlaps tracking pads 214 more than during correct tracking, and more light is reflected by tracking pads 214. The reduced amount of light reflected from tracking pads 212, and the increased amount of light reflected by tracking pads 214 during mistracking to the right, as described in conjunction with FIG. 3c, modifies the tracking signals applied to the servo system. FIG. 3d represents the same subtractor 320 of FIG. 3b, with the positive input signal reduced to a value of 0.2, representing the lesser signal magnitude derived from light reflected from tracking pads 212, and with a greater signal of magnitude 0.8, representing an increased reflection of light by tracking pads 314, applied to the negative input of subtracting circuit 320. The resulting subtracted output signal, used to control the slewing of the optical head or the beams thereof, has magnitude -0.6. FIG. 3e plots the output of subtracting circuit 320 as a result of various degrees of mistracking, which is a conventional plot, well known in the art. The tracking system as so far described may be termed a single-beam tracking arrangement. The servo responds to the magnitude of the light reflected by the beam in such a manner as to close a degenerative feedback loop, for causing recording (or playback) beam 48b and its resulting light spot 310 to track the track path by following tracking pads 212, 214 adjacent to track path 210b. The recording (or playback) beam 310 tends to follow (track) the center of track path 210b.
Playback is accomplished in the one-beam tracking arrangement by operating with the read beam set (often to a lower power) for recovering the data from the recorded track path, and using the same servomechanism to cause the read beam to track the recorded track path, which lies between the tracking pads. If the prerecorded or preformatted tracking pads are erasable, they may be recorded over during the recording operation, as a result of which playback tracking may not be possible, since some of the tracking pads may be completely overwritten. Consequently, the tracking pads may be preformatted on the disk in a non-erasable manner, i.e. in a manner which cannot be obliterated by the subsequent normal optical recording. A corollary of nonrecordable tracking pads is that, if the record beam passes over a tracking pad during the recording process, information or data will not be recorded at that location on the track path. In principle, the recorded track path will have only its edges overlying the tracking pads, and the recorded signal should experience only a slight diminution of amplitude at the location of a tracking pad. However, the tracking servosystem may allow transitory deviations of the recording from the desired track path, so that the entirety of the recorded track path may occasionally deviate from the desired track path lying between the two sets of tracking pads, and completely overlie a tracking pad. Under such conditions, the information would not be recorded onto the disk, and would be lost. The possible loss of information occasioned by overwriting the non-writable track pads is overcome by discontinuous information recording, namely by recording information only along the track path at locations lying between the tracking pads. FIG. 4 represents a track path 210b with tracking pads 212, 214, which is recorded with information or data only at discontinuous track portions 410, which are represented in FIG. 4 by hatching. Recorded track portions 410 are caused to lie between tracking pad sets 212, 214 by operation of the tracking servosystem described above, in response to light reflected from tracking pads 212, 214, all in known manner. A recurrent portion 490 of track 210b has length L.sub.1, which encompasses adjacent tracking pads 212 and 214. Portion 490 is not recorded with information or data.
Other tracking arrangements are known, including one which uses beam splitters to split a light beam from a light source to form three spaced-apart beams with a fixed separation, a center one of which is modulated for recording, or which senses the recorded data for playback. The two outside beams of a three-beam system are substituted for the left and right tracking pads of the track path. Techniques for splitting the beams, for sensing recorded information for playback, and for tracking by use of a servo controlled by light reflected from tracking pads, are well known in the art. One of the disadvantages of the three-beam tracking arrangement is that, when a single light source is used to generate the three beams, less power is available for each of the three beams, which may reduce the power in the record beam, and thereby reduce the recording bandwidth (by requiring a longer dwell to achieve sufficient power for recording at a data spot), or the lesser power in the tracking beams may cause noisy tracking. Another tracking system uses a single beam, which is dithered across the track path. The decrease in reflected light amplitude as the beam tends to leave the track path provides the information required for a tracking servo to, on average, urge the beam toward the center of the track path. Dither-type tracking allows maximum beam power because only one beam is used, but has the disadvantage that a preformatted track path must exist on the disk before recording can begin, and it guarantees that the information signal-to-noise ratio (SNR) will change at the dither rate, because the tracking servo depends upon periodic mistracking in order to generate its input signals.
When multiple simultaneous tracks are required because of a large information bandwidth to be recorded or played back, tracking is, of course, still required. The optical head may produce a fixed set of record/playback light beams, radially spaced apart from each other by the track pitch, and therefore only one beam of the set of beams must be forced to follow a track by a tracking servo, because the remainder of the set of record/playback beams will track with the one which is servoed. When dither-type tracking is used in a multitrack situation, all of the tracks suffer from the periodic degradation of SNR, since all of the tracks move together. The three-beam tracking arrangement may be used with one of the multiple tracks, but has the disadvantage of differences among the bandwidths of the different tracks.degree.
Improved optical disk multitrack tracking is desired.